1) Field
Embodiments of the present invention generally relate to plasma processing equipment, and more particularly to methods of controlling temperatures during processing of a workpiece with a plasma processing chamber.
2) Description of Related Art
In a plasma processing chamber, such as a plasma etch or plasma deposition chamber, the temperature of a chamber component is often an important parameter to control during a process. For example, a temperature of a substrate holder, commonly called a chuck or pedestal, may be controlled to heat/cool a workpiece to various controlled temperatures during the process recipe (e.g., to control an etch rate). Similarly, a temperature of a showerhead/upper electrode or other component may also be controlled during the process recipe to influence the processing. Conventionally, a heat sink and/or heat source is coupled to the processing chamber to control the temperature of a chamber component at a setpoint temperature. A controller, such as a PID (proportional-integral-differential) controller is employed for feedback control of the heat transfer between the temperature controlled component and the heat sink/source. Steady state errors occur with simple feedback control unless a large enough integrator is used. In simple Proportional control there is always steady state error in the presence of external disturbance (unless proportional gain is infinity). However use of large integral control results in poor transients with large overshoots and requires long settling times. Unlike mass flow controllers (MFCs) which have short response times requiring only a few seconds to converge to a setpoint, chamber component temperatures, such as an electrostatic chuck or showerhead temperature, may require 30 seconds or more to stabilize when perturbed during a plasma process due to the significant thermal mass of the chuck, etc. As such, to most quickly compensate for disturbances, large integrator values may be utilized in the feedback controller which has the undesirable side effect of making the temperature control more unstable.
Furthermore, to accommodate increasingly complex film stacks, many plasma processes expose a workpiece to a number of sequential plasma conditions within a same processing chamber. Operations in such in-situ recipes (performed within a single manufacturing apparatus rather than in separately tuned systems) may require temperature setpoints spanning a wide range which introduces a non-linearity into the system such that perturbations or disturbances occurring while the temperature is near a system extreme render response times intolerable.
A temperature control architecture for a plasma processing chamber that improves stability and provides for improved transient response and small steady state error when perturbed is therefore desirable.